WordPress RSS date time
<lastBuildDate>
<?php echo mysql2date(' r', $post->post_date, false); ?>
</lastBuildDate>
Another example:
<pubDate>
<?php echo mysql2date( 'D, d M Y H:i:s +0300',
get_post_time( 'Y-m-d H:i:s', true ), false ); ?>
</pubDate>